This invention relates generally to procedures and machines for incremental printing of text, graphics or photograph-like images on printing media such as paper, transparency stock, or other glossy media; and more particularly to collection and use of information about the condition of printing elements, in preparation of printmasks for multitask printers.
Incremental printers form images through an elegant overlay of micromechanical, electronic, chemical, liquidic and capillary phenomena. It is accordingly in the nature of such systems to be vulnerable to infinitesimally fine perturbations from manufacturing perfection, and so they are.
In fact, it is desirable that incremental printing systems be able to operate satisfactorily with the poorest possible degree of manufacturing perfection, since such apparatus is maximally economical. From its beginnings, the incremental-printing art has undergone continuous struggles with several successive stages of image-quality difficulties, all arising from sensitivity of this basic methodology to fine irregularities in the apparatus.
Meanwhile, in a quest for greater image throughput, printing-element arrays have been made both progressively larger in overall length and progressively finer in resolution. This evolution has continued to aggravate the image-quality problems that arise from tiny imperfections.
(1) Primitive Banding
Earliest apparatus in this field operated on a single-printing-pass basisxe2x80x94that is, one pass of the printhead (printing-element array) over each part of the imagexe2x80x94and suffered from image imperfections, sometimes called xe2x80x9cbandingxe2x80x9d, along lines where successive swaths of marks failed to abut perfectly. Early rounds of refinement in the field therefore focused upon techniques for minimizing the conspicuousness of such interswath seams.
That earliest type of banding presented itself especially in pictorial imagesxe2x80x94i.e. images made up of line drawings, common graphics, or continuous pictures. (Images of lettering, i.e. text, were relatively immune since many choices of text size and spacing could be arranged to print within the height of individual swaths, thus avoiding printing anything across the seams between swaths.)
This banding was mitigated by hiding the edge of each swath within the height of one or more other swathsxe2x80x94or, more specifically, by the introduction of multipass printing: each part of the image receives marks from plural, usually multiple passes of the printhead. Such printing requires allocation of the marks as among the several passes, and this is the function of so-called xe2x80x9cprintmasksxe2x80x9d.
While printmasks have been developed using quite a number of different notations, the underlying idea is quite basicxe2x80x94to define which dots print in which passes. Initially, all the different printmask notations in common consisted of simple, relatively small patterns that were conceptually superimposed over the pixel data of an image.
These masking patterns were stepped or tiled across and down the image to serve in allocating marks for the entire image. Some pioneering patents in the name of Mark Hickman and in the name of Lance Cleveland, among others, canvass the several successively more sophisticated methodologies that developed in this era, in the late 1980s and early 1990s.
(2) Progressively Subtle Forms of Banding
Unfortunately, while multipass printing was reasonably effective in concealing the interswath-seam type of banding it also introduced new and progressively more resilient forms of banding that continued to challenge designers in this field for several years.
Because the earliest masks were regular, they tended to visually beat against the pixel-structure periodicity of images, or against features of the images themselvesxe2x80x94producing repetitive moirxc3xa9-like patterns that were often very conspicuous. In response, pseudorandomness was introduced into a later generation of printmasks.
Surprisingly, these patterns tooxe2x80x94being small and repetitivexe2x80x94rather than eliminating patterning altogether, instead yielded bizarre shapes variously described as xe2x80x9cwormsxe2x80x9d or xe2x80x9corganxe2x80x9d shapes that seemed to crawl repetitively across the images, particularly in midtones. These stubborn artifacts, a new and different kind of banding, in general represented shapes entirely unrelated to image features.
It was thought that actual randomness would dispel these obtrusive forms, but they later remained even when true randomness was introduced into printmasks. The problem was that masks were still smallxe2x80x94and tiled across and downxe2x80x94so that creeping kidneys, and so forth, continued to be generated repetitively and with fixed periodicity, and thus to be conspicuous.
These types of banding effects also persisted when masks were made larger, up to 2 cm (about xc2xe inch) wide and tall. In addition, with truly random masking there seemed to appear a new, different kind of image defect: randomness-generated granularity.
Some workers noted that banding was generated basically because individual printing elements were mispointed, or firing more heavily or lightly than nominal, or simply had failed. This observation led artisans to look for relatively simple solutions in the nature of identifying misbehaving elements and straightforwardly reassigning the printing tasks of each degraded element to some other element.
This simple-reassignment type of system turned out to be capable of improving image quality very significantly, but only for a short time. When all the tasks of a failed element were transferred to some nominally healthy surrogate, that healthy element was thereby called upon to do double duty.
In many or most cases this two-times normal loading on the healthy (but, after all, not really new) element accelerated its aging and deterioration. Reassignment of its double-overload tasks to yet another element was soon required, leading to triple overload of that backup unit. Skilled workers in this field could then see that simple reassignment would provide no longterm solution, within the life of a printer.
(3) xe2x80x9cShakesxe2x80x9d
The next major developments in masking for incremental printing were introduced by Joan-Manel Garcia, whose patent documents have been mentioned earlier. He devised a powerful conceptual construct within which to generate randomized masks for very large photograph-like images automatically.
Garcia""s mask-generating procedures, which he named xe2x80x9cShakesxe2x80x9d, can operate in the fieldxe2x80x94and on the fly if desiredxe2x80x94obviating the need for dedicating large amounts of nonvolatile data storage to hold factory-computed large masks. This development accordingly in principle removed or greatly elevated an upper size limit for random masks.
He also noticed, however, that masks significantly wider than 3 or 4 cm (roughly 1 to 1xc2xd inch)xe2x80x94even when repetitively tiledxe2x80x94no longer led to noticeable banding. Garcia explained this observation in terms of spatial frequencies to which the human eye is sensitive.
Hence Garcia""s random masks needed only be made about 3 or 4 cm wide (and tall) to avoid perceptible repetition, and these sizes were very readily within the ability of his algorithms to produce. Even if present, the so-called xe2x80x9cwormsxe2x80x9d are much less conspicuous when the stepped masks are of these larger sizes.
Further, into this methodology Garcia integrated important observations about perceptible granularity. More specifically, he recognized that graininess in images can be generated as a form of signal noise in the masking process, particularly as a result of randomness.
Hence Garcia was able to include in his formulations a balancing of desirable randomness that minimizes banding vs. undesirable randomness that raises perceptible grain. Garcia and his colleagues also introduced waysxe2x80x94including one formulation which they called xe2x80x9cpopupxe2x80x9d masksxe2x80x94to mitigate the relatively large amounts of computation entailed in Garcia""s classical Shakes procedures.
Another particularly beneficial property of the Shakes techniques is that they attack not only the first-generation problem of failed printing elements but also the second and third waves of failure generated by the simple-reassignment technique discussed above. When an element is found to be malfunctioning, Shakes and its popup variant refrain from passing element-task overloads from one victim to the next.
Instead they recompute the overall allocation of printing burden for the entire reduced population of elements. In doing so they also take into account the type and severity of malfunction.
Shakes and popup masking thus represent the highest forms of the masking art heretofore, particularly for large, photo-like, highest-quality images. Such applications of incremental printing represent a very specialized kind of work, for which some relatively large amount of preliminary processing time is entirely acceptable.
In particular the real-time algorithmic investment represented by the popup variant of Shakes is very plainly justified by the enhanced image quality and controllability it achieves for large photo-like images. Within that specialized domain, Garcia""s formulations are extremely effective, and it is by no means intended to minimize his accomplishments.
(4) Masking for Multitask Printers
That achievement does not necessarily extend to all kinds of incremental printing, particularly to some types of printers that perhaps do not enjoy the luxury of specialization. Some such printers are required to service an incremental-printing market in which any two successive printing tasks are likely to be highly divergent.
While one image to be printed may be photograph-like, the next may be commercial graphics or an engineering diagram; and the following one may be a text document. Interestingly, printers expected to operate in such a dynamic environment may also be required to handle all these different job types with faster turnaround time than a specialized machinexe2x80x94and despite all these more-stringent demands may be regarded as near a lower end of a product line than the specialized unit.
Hence even the relatively modest cost of memory for Shakes or popup-style masking may not be available in a multitasking printer. Analogously the relatively modest processing of the popup masking may require undesirably long processing times using a slower processor provided in a xe2x80x9clower endxe2x80x9d multitasking machine.
As suggested above, Shakes and popup masking operate according to a highly elaborated regimen, based upon a preestablished abstract mathematical model of masking circumstancesxe2x80x94taking into account a number of considerations that are applicable in general but perhaps do not come into play in the majority of cases. This regimen has many benefits including great power, and adaptability to newly recognized print-quality defects and complicated masking situations, as well as to new kinds of images, and it is very efficient in the large-image, photo-quality environment.
In a multitasking environment its overall efficiency and perhaps even its applicability may be less clear. The Shakes/popup regimen does call for a certain level of commitment from the host system, in terms of data storage as well as computational capability.
Such commitment is well balanced with resulting benefits when the condition of each printing element is relatively stable over time. That assumption is valid in the context of printing many large, photograph-like images but often fails in the context of a multitask printerxe2x80x94in part because of different ink formulations (particularly dye- vs. pigment-based inks).
In multitask machines, serving different markets simultaneously, ink usage is often heavy and therefore printing-element degradation fast and erratic. In these systems contamination may move among interconnected printing elements and even among different printheadsxe2x80x94as for example in the form of printing-medium fibers trapped in printhead service-station modules.
Consequently plot-to-plot testing in this environment is often more appropriate than the usual schedule for specialized operation of large-format machines. Those units are checked only when pens are changed, and perhaps periodically (e.g. weekly).
What would be ideal for the multitask environment is a much more simple procedurexe2x80x94to prevent image-quality degradation due to actually failed printing elements, and as in Shakes to avoid passing overload down the printing-element line until overall failure is precipitated, but yet at the same time to provide efficiencies specially tailored to multitask situations. In other words, what is wanted for use in a multitask printer is a procedure that is simultaneously (1) more lean, or spare, in use of operating resources; and yet also (2) better able to rapidly and reliably manage masking for multiple project typesxe2x80x94with perhaps an emphasis on small format and a slightly lower range of image quality.
In particular the ideal multitask masking procedure should also be able to quickly and easily assess and deal with instability and even erraticism in the conditions of individual printing elements. These streamlined properties are needed in both the mask-generation procedure considered in isolation and also the overall process of masking and then printing. Heretofore no such ideal multitask masking procedure has appeared.
(5) Assessing Printing-Element Condition
From the foregoing it may be divined that a strongly felt need of mask generation and printing, in multitask printers, is a correspondingly streamlined methodology for profiling the health of a printhead. Neither the Shakes/popup strategy nor any other masking system that aims to cover for failed printing elements can be effective without first assessing which elements are in fact failed.
For the multitasking environment, this chore of evaluating individual printing elements takes on a peculiar, unusual propertyxe2x80x94since this environment is notorious for exhibiting high levels of instability in indicated printing-element health. Some more-detailed understanding of the reasons for instability will be introduced later in this document, particularly in terms of so-called xe2x80x9csensor noisexe2x80x9d.
An effective evaluation must account not only for the current or instantaneous condition of each print element but also for the degree of stability of that condition as measured. In this regard, the Shakes system of printing-element health assessment adopts a generally accepted approach of measuring and storing multiple historical health values for each element: whenever the overall printer operating protocol calls for measuring print-element health, the current measured value is appended to the archive of previously stored values, on a first-in/first-out basis.
The system then later, when masking is required, retrieves and derives from these data necessary measures of printing-element conditionxe2x80x94and stability as well, if required by the particular system. With increasing print-element array length and progressively finer array resolution, this conventional approach demands dedication of progressively larger amounts of memory to storingxe2x80x94overall, for an entire arrayxe2x80x94very large numbers of sequential test results.
This is not all: these many individual test data, stored raw, must then be processed before use to determine needed measures of condition and stability. Hence not only memory but also real time (that is to say processing time in real time, while the printer and operator are both waiting to print out an image) must be sacrificed to the prior-art strategy for acquiring and using print-element health data.
(6) Conclusion
As this discussion shows, with respect to multitask printing the prior art imposes relatively high demands upon memory and processing time for automated field preparation of printmasks. These time and memory demands continue to impede achievement of excellent but economical multitask incremental printing. Thus important aspects of the technology used in the field of the invention are amenable to useful refinement.
The present invention introduces such refinement. In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits.
In preferred embodiments of a first of its facets or aspects, the invention is a method of revising a plural-pass printmask when one or more malfunctioning printing elements have been identified, in a scanning incremental printer. The method includes the step of, for each identified malfunctioning elementxe2x80x94and for at least one identified-element nonzero entry in the maskxe2x80x94at a particular pixel position, finding zero entries in the mask for other printing elements that service the particular pixel position.
The method also includes the steps of, still for each identified malfunctioning element: selecting one of the other-element zero entries for replacement by the identified-element nonzero entry; and replacing the selected zero entry by the identified-element nonzero entry. For purposes of this document, the terms xe2x80x9creplacementxe2x80x9d and xe2x80x9creplacingxe2x80x9d of a selected zero entry by a nonzero entry for an identified element are not meant to imply that specific numbers of the nonzero entry necessarily are literally copied or moved into the zero entry.
Depending upon the masking format and notations in use, it is possible that such literal replacement may occur; but this is not the general case. More meaningfully and more generally, what is replaced is the intended function, or functionality, of the nonzero entry in eventually commanding the printing of a particular mark or marks at a particular pixel position.
(This first major facet of the invention does not extend to actual printing, but rather as stated above is simply a procedure for revising a mask. Nonetheless a basic assumption is that the revision is for purposes of preparing a mask that in due course will acquire utility when it is used in printing; and this is the perspective in which the above-mentioned xe2x80x9cfunctionalityxe2x80x9d develops.)
It is that functionality which is displaced from one printing pass (where it would require use of a printing element known to be malfunctioning) to another pass (where the implicated printing element is in good condition). In notational terms this replacement or displacement of functionality may take any one of a great variety of forms.
The foregoing may represent a description or definition of the first aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
This extremely simple procedure analyzes actual mask entries already established for the actual image that is about to print outxe2x80x94rather than analyzing, in a complicated way, preestablished abstractions of possible masking conditions. This approach enables a printing apparatus to prevent image-quality degradation due to actually failed printing elements while at the same time avoiding the previously suggested inefficiencies of complicated, abstract processing.
Although the first major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the method further includes the step of deleting the identified-element nonzero entry.
Another basic preference addresses the situation in which at least one of the one or more identified elements is functionally impaired but not completely inoperative. In this case, subsidiary preferences are, respectively, that for at least one impaired element, the identified-element nonzero entry is not deleted; and that for at least one impaired element, the replacing step is performed for fewer than all the identified-element nonzero entries.
If this last-mentioned subpreference is observed, it is still further preferred that the one or more impaired elements be assigned numerical weights according to degree of impairment, and that a proportion of the weighted elements for which the replacing step is performed be controlled by the numerical weights.
Yet another basic preference is that the selecting step be controlled by conditions near the particular position. In this case, one subpreference is that the selecting step include applying the conditions to minimize adjacent drop deposition. If this subpreference is in effect, then a nested subpreference is that the applying include minimizing consecutive dot formation along a pixel row.
Another subpreference (within the nearby-conditions basic preference) is that the finding or selecting step, or both, include a search through the other-element zero entries, along a sequence. In the case of this subpreference, then it is still further preferred that the selecting step include checking conditions only upstream from each other-element zero entry, along the search sequence.
Still another basic preference is that each of the other elements is at a position distant from the particular element by a number of elements equal to:
xc2x1nxc2x7a,
where n=cyclical pass-number increment, and
a=printing-medium advance, counted in number of printing elements, between scanning operations.
A still-further basic preference is that the mask is a prequalified mask. If this is so, then preferably the mask is an eight-pass mask; and in turn preferably the mask is a two-bit mask; and further in turn the printing elements are in an array, and the mask is substantially the height of the array; and yet further in turn preferably the mask is 288 rows tall and 128 columns wide.
If the mask is prequalified, it is also preferably in binary form and is ANDed w/binary data to create a resultant binary-form printing specification. Here preferably the printing specification includes resultant binary digits, each digit representing one individual dot without binary-position implication.
Yet one other basic preference is that the finding or selecting step, or both, include choosing other printing elements that have not been identified as malfunctioning. Also as noted above it is preferred that the first main aspect of the invention be used in combination together with other major facets introduced below.
In preferred embodiments of its second major independent facet or aspect, the invention is a method of revising a plural-pass printmask when one or more malfunctioning printing elements have been identified, in a scanning incremental printer. The method includes the step of, for each identified malfunctioning element, selecting a prequalified printmask.
It also includes the step of modifying the mask to replace exclusively entries for malfunctioning printing elements. This modifying includes reassigning entries for each malfunctioning element to plural other elements respectively.
The foregoing may represent a description or definition of the second aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, as described in the xe2x80x9cBACKGROUNDxe2x80x9d section of this document, early methods tend to follow two very divergent philosophiesxe2x80x94skipping over, and missing, an especially favorable middle ground. Thus one earlier algorithm simply reassigned all the tasks of a malfunctioning printing element to some other element.
In this first simple-reassignment type of earlier algorithm under consideration, that element unfortunately is thereby double-overloaded and tends to fail relatively soon afterxe2x80x94and all of its already-overloaded assignments are then transferred to yet another element, which is then triply overloaded. Plainly the ill effects can cascade into an avalanche of printing-element failure, shortly disabling the entire printhead.
Recognizing this problem, other earlier methods ambitiously establish a very powerful system, based on a relatively abstract concept, mainly for printing large photographic-quality images and capable of dealing with complex masking situations and problems. Those earlier formulations require significant computational overhead and data-storage commitmentsxe2x80x94all very well justified by:
(1) the relative stability of either function or malfunction by each printing element; and
(2) the resulting efficiencies in dealing with very large images, very high quality, and masking-problem complexities.
In this second group of earlier methods, the general approach when confronting a failed printing element is to stop and design a wholly new printmask to service the reduced complement of elements that remains. Naturally this total-redesign approach can work very well, redistributing the printing burden very equitably and carefully among all the working elements while observing all the best constraints for avoiding ink coalescence and other image-quality defects. It is, however, burdened by the high overhead mentionedxe2x80x94especially onerous for multitask systems.
The second aspect of the present invention, by comparison, undertakes to avoid the simple-reassignment problem of overloading elements that receive reassignments from failed elementsxe2x80x94but also assumes a much more humble posture and undertakes a far more modest task than the total-redesign approach. That task is to handle quickly but surefootedly the masking requirements for many different kinds of projects, but most generally smaller-format images and at a somewhat lower range of image-quality demandsxe2x80x94particularly a multitask printing system in which malfunction or function of each printing element is not at all stable but rather is transitory and indeed erratic.
In this multitask environment the high investment of a powerful masking system would represent inefficiencies. The extremely simple, ad hoc approach of the second facet of the invention yields both efficiency and robustness.
Although the second major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the reassigning step includesxe2x80x94for a particular printing element whose entries are to be reassignedxe2x80x94attempting to reassign each entry to a different one of those plural other elements, respectively.
As a result, subsequent printing-element degradation caused in printing at the specific pixel positions tends to be distributed relatively broadly among the plural other elements rather than assigned to a relatively few other elements. The term xe2x80x9cattemptingxe2x80x9d is used above because various factors limit the ability of the system and the algorithm to actually reassign every problem entry to a respective different element.
For example, the procedure may be designed to consider only prospective reassignmentsxe2x80x94i.e., reassignments to pixel positions that have not yet been analyzed, or not yet printed. Literally reassigning every malfunctioning-element entry to a different element in some cases might require going back to the beginning of the entire procedure and beginning again with previously processed pixelsxe2x80x94which, for this example, would violate the procedural design.
Hence in such cases the algorithm would xe2x80x9cattemptxe2x80x9d to reassign to different elements in the sense of doing so until it was no longer possible to so proceed within the stated design constraint. It can be written to thereafter attempt to assign perhaps only a maximum of two entries to each of the other elements, and so forth.
In preferred embodiments of its third major independent facet or aspect, the invention is a method of incremental printing with a revised plural-pass printmask when one or more malfunctioning printing elements have been identified, in a scanning incremental printer. Thus this method is capable of extending beyond that of the first main aspect of the invention, which was only a method for revising a mask.
The present method includes the steps of, for each identified malfunctioning element:
for at least one identified-element nonzero entry in the mask, at a particular pixel position, finding zero entries in the mask for other printing elements that service the particular pixel position;
selecting one of the other-element zero entries for replacement by the identified-element nonzero entry;
replacing the selected zero entry by the identified-element nonzero entry; and
iterating the finding, selecting and replacing steps.
Earlier discussion of the terms xe2x80x9creplacementxe2x80x9d and xe2x80x9creplacingxe2x80x9d is applicable to those terms as used here tooxe2x80x94but that discussion of functionality is potentially more directly meaningful in this context, since this method can encompass a step of actual printing.
The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention actualizes, in the context of actual printing of an image, all of the potential benefits of the first facet of the invention as described above for simply revising a printmask in preparation for printing. In other words, the efficiency of the present invention is very high, for printing images in a multitasking environmentxe2x80x94and correspondingly in printers whose printing elements do not perform in a stable way but rather have dynamically changing condition.
Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the present method includes printing an imagexe2x80x94using the mask revised as set forth above.
Another preference is that the iterating step include continuing for roughly twenty percent of all printing elements in the printer, or fewer. An alternative preference is that the iterating step include continuing for roughly ten percent of all printing elements in the printer, or fewer.
In preferred embodiments of its fourth major independent facet or aspect, the invention is a method of storing information about printing-element condition, for use in incremental printing. The method includes the step of, on successive occasions, testing printing elements.
It also includes the step of, for each element, on substantially each of the successive occasions, keeping a record of two pieces of information: (1) at least one recent test result; and (2) a number which is a cumulative measure of performance stability.
The foregoing may represent a description or definition of the fourth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, by storing a cumulative measure of stability together with a recent test result, the invention enables assessment of, first, the degree to which the recent test result is representative of the actual condition of the element; and, second, therebyxe2x80x94if the stability indication is highxe2x80x94an estimate of the probable condition of the element. Furthermore, if the stability indication is low, then the invention yields directly an assessment of the condition of the element as unstable.
In this way, this fourth facet of the invention enables automatic processes in a printer to classify a printing element as usable or not, without devoting a large amount of memory to storing many sequential test results. This aspect of the invention operates on a principle of determining and storing a qualitative conclusion from test data, rather than the raw data themselves.
This facet of the invention thus extracts a great deal more value from minimal memoryxe2x80x94and with minimal processing, too, since the conclusion is already preformulated for use. In short this fourth facet of the invention enables the first three aspects of the invention to function optimally, with a very small dedication of memory and processing overhead to keep track of printing-element condition.
Although the fourth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the record-keeping step includes, on substantially each of the successive occasions, incrementing the stability-measure number in one direction if a current test result and the recent test result are the same, and in an opposite direction if the current and recent results are different.
In this way the cumulative stability-measure number trends to one extremum value for each element that is longterm-stable and to an opposite extremum value for each element that is longterm-unstable. In other words, if an element has longterm stability, the cumulative measure trends to a particular one extremum value. If the element can be characterized as unstable, on a longterm basis, then the cumulative measure trends to an opposite extremum value. Hence, again without storing a sequence of numbers but instead storing just two numbers, this ingenious procedure is able to generate an incisive indicator of fundamental printing-element condition.
In event that this basic preference is in effect (i.e., of crosscomparing the current and recent results to determine which way to increment the stability-measure number), then a specific subpreference is that the incrementing in one direction include positively incrementing the stability-measure number; and the incrementing in an opposite direction include decrementing the stability-measure number. In this way, for a stable element the cumulative stability-measure number trends to a maximum value; and for each unstable element, to a minimum value.
Another subpreference (to the basic crosscomparison preference) is that the incrementing include adjusting the number by a respective amount that depends on current value of the number. In this case it is still-further preferred that the amount of adjustment toward values representing low stability be permitted to be larger than the amount of adjustment toward values representing high stability.
Some additional subpreferences to the basic crosscomparison preference are:
that the method further include the step of, before first testing of a printing element, initializing the number for that element to a value representing substantially maximum stability;
that the number be a four-bit binary number, thus ranging from zero through fifteen, and the intermediate value be roughly eight; and
that the test result be an indicator of nozzle failure or successful firing in a single test; and that the method further include the step of, for each element on substantially each of the successive occasions, keeping a record of:
at least one additional less-recent test result; and
at least one value which serves as a measure of previously-failed-nozzle recoverability.
One further basic preference to be mentioned is that the record-keeping step include, on substantially each of the successive testing occasions:
incrementing the stability-measure number to maintain a count of the number of test results identical to a test result immediately preceding a current test result; and
clearing the stability-measure number to zero if the test result immediately preceding a current test result differs from a next-earlier test result.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: